High-purity silanes having two or more different halogen moieties (referred to herein as “mixed-halogen halo-silanes”) are useful sources of atomic silicon and halogens in a variety of applications. For example, trichloro-fluoro-silane (SiFCl3), dichloro-difluoro-silane (SiF2Cl2), and chloro-trifluoro-silane (SiF3Cl) has been used as a source of high-purity silicon in the preparation of semiconductor materials and communication-quality fiber optic materials. In addition, it has been recently reported that trifluoro-iodo-silane (SiF3I) is particularly amenable to laser isotopic separation, providing isotopically pure silicon for use in the preparation of isotopically pure silicon wafers. Likewise, fluoro-iodo-silanes have been used as a source of high-purity iodine in the preparation of iodine gas lasers. Generally speaking, to be of use as an element source of silicon and halogens, the mixed-halogen halo-silane must be high purity. The term “high purity” as used herein refers to a composition comprising at least 99% by weight of one silane species and less than 0.5% by weight of non-silane impurities.
There are a number of conventional approaches for preparing high-purity mixed-halogen halo-silanes. One approach involves treating elemental silicon with mixtures of elemental halogens. For example, silicon may be treated with a mixture of F2 and Cl2 to produce SiF2Cl2. Another approach uses tans-halogenation reactions, in which a silicon compound containing one halide species is contacted with a metal complex containing a different halide species under conditions sufficient to induce halogen exchange between the various species. For example, F3SiCl may be produced from the irradiation of a mixture of SiF4 and BCl3. Other approaches combine these two approaches. For example, elemental silicon may be treated with a halide gas, e.g., Cl2, and a halo-silane, e.g., SiF4, to produce, among other species, a mixed-halogen halo-silane, e.g., F3SiCl.
Although these approaches have been used historically to produce high-purity mixed-halogen halo-silanes, there is a general desire to reduce the costs associated with their preparation. Naturally, reducing the cost of preparing mixed-halogen halo-silanes can lead to a reduction in the cost of producing materials that use them in their preparation. Furthermore, finding new applications for these mixed-halogen halo-silanes depends upon developing economical processes for producing commercial quantities of them. Typically, the most significant barrier to economically producing high-purity mixed-halogen halo-silanes is separating the desired mixed-halogen halo-silane from the other mixed-halogen halo-silane species and byproducts formed during its preparation.
The problems associated with conventional mixed-halogen halo-silane preparation techniques tend to result from the bonding characteristics of the halogens involved. In an Si—X bond (where X is a halogen), the ability to replace one halogen with another by direct halogen substitution tends to follow the order of Si—X bond strength, Si—F>Si—Cl>Si—Br>Si—I. Thus, fluorine can replace chlorine, bromine, and iodine, chlorine can replace bromine and iodine, and bromine can replace iodine. Synthesis of mixed-halogen halo-silanes by preparative schemes based on direct replacement of one halogen species with a different halogen species are problematic because such reactions tend to lead to complete replacement of one halogen with another. Although techniques are often employed to shift the reaction's equilibrium to favor incomplete substitution of one halogen species for another, these reactions nevertheless produce a distribution of perhalogenated products.
Processes that produce a distribution of halosilane species from which a single species must be isolated do not use starting materials efficiently. In addition, because cognate halosilane species boil over a very narrow range, isolation of any one species to achieve high purity requires specialized equipment, such as a high theoretical plate distillation column, which is expensive to obtain and operate.
Therefore, there is a need for a process that uses reactions having high specific conversion of the starting material into a single product species, thereby efficiently using the starting material. The present invention fulfills this need among others.